wildjasmin296741-san, マイケルさん、
こんにちは!
Thank you very much for your help, マイケルさん!
It's really interesting to see the comparision with English
Like マイケルさん wrote, all those three means the "nearest/coming Sunday".
The tricky one is 来週の as we never know "which Sunday" the speaker is refering to.
If the speaker says 来週の日曜日 on Monday, it could be the nearest Sunday, but if this person says
that on Friday, it could be the one after the nearest.
Some even say 来週の日曜日 on Monday, meaning the one in two weeks.
As to 今度の日曜日, it works like English (as マイケルさん explained), but the big difference between
Japanese and English is, even if someone says 今度の日曜日 on Friday, it could mean the nearest Sunday
(the day after tomorrow). Of course, usually people uses the words "tomorrow" and/or "day after tomorrow"
to clarify which Sunday s/he is mentioning.
Some expressions like this can "depend" on the person's idea of (how to count) a week, so
the best is to confirm it with different expression always. It's actually a common pracitce. You can see
how some expressions are very "grey" (requiring clarification to "black" or "white")
Natsuko(奈津子),
Team JapanesePod101.com